A wind turbine known in the art comprises a tapered wind turbine tower and a wind turbine nacelle positioned on top of the tower. A wind turbine rotor with a number of wind turbine blades is connected to the nacelle through a low speed shaft, which extends out of the nacelle front as illustrated in FIG. 1.
Oscillations and vibrations of the wind turbine tower are a problem to a greater or lesser extent for all types of wind turbines but particularly within the art of making active stall controlled wind turbines this problem is profound.
Tower oscillations can e.g. arise from the fact that each time a rotor blade passes the tower, the wind will push slightly less against the tower. If the rotor turns with a rotational speed such that a rotor blade passes the tower each time the tower is in one of its extreme positions, then the rotor blade may either dampen or amplify the oscillations of the tower.
Other sources for tower oscillations are also possible but with active stall controlled wind turbines one source is dominant.
During normal operation of an active stall controlled wind turbine the rotor exhibit positive aerodynamic damping. The tower top is moving around substantially all the time although the movements are small. When the tower top moves upwind the rotor thrust increases, thereby the tower top is pushed back downwind, which dampens the tower vibrations. Similarly, when the tower top moves downwind the rotor thrust decreases thereby the tower top is pushed back upwind, which again dampens the tower vibrations.
But the rotor aerodynamics of an active stall controlled wind turbine is non-linear. On a thrust versus wind speed curve such a rotor will exhibit a relatively small region of negative slope, which results in that under certain conditions the rotor provides negative aerodynamic damping. When the tower top moves upwind the rotor thrust decreases, thereby the tower top is pushed further upwind, which amplifies the tower vibrations. Similarly, when the tower top moves downwind the rotor thrust increases, thereby the tower top is pushed further downwind, which again amplifies the tower vibrations.
In the worst case this negative aerodynamic damping can make the tower oscillate to a degree that the tower or other parts of the wind turbine is damaged, to a degree that the life of the tower or other parts are severely reduced or to a degree that the efficiency of the wind turbine is reduces.
One way of dealing with this problem is to take different measures during the design phase of the wind turbine e.g. by making the tower stiffer, by designing the blades differently, by reducing the weight of the nacelle and rotor or other. But these measures often collide with other desired qualities of the wind turbine, such as low production cost, high efficiency and other.
Another way is to simply shut down the wind turbine when critical tower oscillations is detected and then start up again, when the wind turbine is completely shut down or when certain critical conditions has changed—such as when the wind speed has changed. This method is under certain circumstances very effective in eliminating or damping the tower oscillations but it takes a long time to shut down and restart a wind turbine, hence the overall power output of the wind turbine is reduced. Furthermore, it increases the wear on many parts of the wind turbine to stop and start frequently, and to increase the life of the wind turbine stopping and starting should therefore be kept to a minimum.
Other ways of dealing with this problem has therefore been developed and one of these is disclosed in European patent application No EP 1 008 747 A2. This application discloses a way of damping tower movements by means of a dampened pendulum suspended in the tower. But these types of dampers have to be very large and heavy to dampen tower oscillations efficiently in a large modern wind turbine and they are both expensive, they add unwanted weight to the tower or to the nacelle—which the tower has to be designed to carry—and they take up space in the tower or in the nacelle.
An object of the invention is therefore to provide for a technique for damping or eliminating tower oscillations in the tower of an active stall controlled wind turbine, which do not present the mentioned disadvantages.
Particularly, it is an object of the invention to provide for a simple and cost-efficient technique for damping or eliminating tower oscillations in the tower of an active stall controlled wind turbine.